Melting furnace



Feb. 25, 1969 K. s. SNOW MELTING FURNACE INVENTOR Karl 5. Snow Y g.Agent Feb. 25, 1969 K. s. SNOW 3,429,564 I MELT ING FURNACE Filed Aug.2. 1965 Sheet 2 of 2 INVENTOR Karl 8. Snow United States Patent ClaimsABSTRACT OF THE DISCLOSURE A furnace having a sealed shell and means forevacuating said shell. A rotatable hearth having at least one moldcavity within said shell and an adjustable plasma torch having adischarge nozzle located within said shell to direct the eflluent ontosaid hearth.

This invention relates to a furnace for melting buttons or small ingotsof metals. It is particularly adapted for melting refractory metalswhichlrequire metling at are temperatures and which are also reactive atelevated temperature requiring melting in a protective or inertatmosphere.

Small button furnaces, as they are called, are most often used to meltsmall circular discs' or buttons of experimental alloys or for meltingsamples of metals for analytical purposes. In addition to discs,pillow-shaped small ing'ots, as well as bars, may be produced. An arcfurnace has most often heretofore been employed using a non-consumabletungsten electrode to melt a small amount of metal in a cavity in awater-cooled hearth. The furnace may be maintained under vacuum orfilled with inert gas to prevent contamination of the melted metal byatmospheric gases, particularly oxygen and nitrogen.

It is difl'lcult, however, to obtain desired purity of melted metal insuch a tungsten electrode furnace. A melting arc is often erratic,causing spattenof metal from the mold cavity, and this may contaminatemetal in adjacent cavities if a multiple cavity hearth is used. Of moreimpor tance is the tendency of metal to spatter onto the tungstenelectrode itself, and when this becomes molten or falls off as solid, itcarries tungsten contamination with it. Contamination from the tungstenelectrode is a serious disadvantage of this type of melting.

Summarized briefly, the furnace of this invention operates with a heatsource providing a steady and controllable stream of arc plasma. Thisavoids problems associated with a melting arc as heretofore known andused. It avoids possible contamination by tungsten from the electrodeand enables a plurality of cavities to be arranged in close proximity ina hearth without danger from contaminating spatter. Use of a plasma jettorch provides a stable eflluent that can readily be directed onto asample or mass of metal to be melted. The torch operates with a tungstenelectrode, but this is placed up inside a barrel and protected by thenozzle as well as the high speed plasma efiiuent. Under theseconditions, spatter onto the tungsten electrode is eliminated as istungsten contamination in the melt from this source.

The steady and readily controlled plasma efliuent also eliminates orreduces spattering from the molten pool of metal being melted.Therefore, closely spaced cavities in a multiple cavity hearth may beemployed; and only minimum, if any, shielding between cavities isrequired.

The plasma jet torch requires an inert gas flow to provide a hot,gaseous efiluent. This results in a continuing inflow of inert gas intothe furnace during operation, and provides an ideal atmosphere in thefurnace for melting reactive metals. Thus, the required inert gasatmosphere 3,429,564 Patented Feb. 25, 1969 is automatically maintainedin the furnace during opera tion. The furnace of this invention will bemore clearly understood by reference to the following more detaileddescription thereof, and to the drawings, in which FIG. 1 shows a sideview, partly broken out, of a furnace embodying features of thisinvention;

FIG. 2 shows a horizontal cross section of the furnace of FIG. 1 takenalong the line 2-2;

FIG. 3 shows in detail the furnace view-port and its protective screen;

FIG. 4 shows in detail the furnace hand-hole and cover;

FIG. 5 shows a cross section of the hand-hole of FIG. 4 taken along theline 5-5; and

FIG. 6 shows details of the plasma jet torch.

Referring now particularly to FIGS. 1 and 2, the furnace comprises asupporting base composed of uprights 10, lower horizontal bars 12 andupper horizontal bars 14. On the top-of upper horizontal bars 14 isattached furnace lower plate 16 to which the furnace shell 18 isattached and sealed thereto by O. ring 20. Furnace shell 18 is generallyhemispherical at its top and encloses the furnace working parts, as willbe hereinafter described in detail. It is constructed so as to begas-tight or sealed to prevent entry thereinto of atmospheric gases.

Vertically mounted drive shaft 22 passes through furnace bottom plate16, its passages being sealed by rotatable seal and thrust bearing 24.Drive shaft 22 is hollow, carrying a central water inlet tube 26 towhich is connected at its bottom cooling water supply line 28. Theannular space between bottom pipe 26 and the interior surface of shaft22 provides a channel for outflow of cooling water, which may bediscarded to a suitable drain through drain pipe 30.

Vertical shaft 22 is is rotated by means of encircling gear 32, fixedlyattached to the exterior surface of vertical shaft 22, which meshes withworm 34, which is rotated at desirable speed by speed reducer 36connected to the output shaft of motor 38. Support bracket 40 supportsmotor 38 in proper position for connection of the drive train described,and is itself supported on horizontal bars 12.

Rotation when desired of shaft 22 is controlled by pedal switch 42connected through lead 44 to a suitable source of electric powerindicated as power cabinet 45, in turn connected to motor 38 throiighlead 46.

Fixedly attached to thev'top of vertical shaft 22 is rotatable hearthindicated at 48 having base 50. Rotatable hearth 48 is provided with anupper heavy circular plate 52, preferably fabricated of copper to takeadvantage of this metals heat transfer characteristics. Upper plate 52is attached to base 50 by edge plate 54 maintained in tight contact topand bottom by peripheral bolts 56. The top and bottom elements of hearth48 are arranged so that there is spaced for circulation of cooling watertherebetween, such cooling Water being introduced through pipe 26 anddistributed to the edges of hearth 48 by subsidiary pipes 57, the returnwater flowing through the open space back down through hollow verticalshaft 22 and out drain pipe 30.

Upper plate 52 of hearth 48 is provided with a plurality of indentationsor open-top cavities as at 58 into which metal may be melted into ingotsor buttons. Cavities 58 may be of various shapes to provide the desiredbutton or small ingot shape, as will be seen more clearly in FIG. 2.These, for example, may be designed as pillow block cavities 58a,circular cavities for producing metal buttons, as at 58b, .or barcavities for producing elongated metal shapes, as at 580.

In the top of furnace shell 18 is arranged loading port 60 which isclosed by provision of cover plate 62. Clamps 64 maintain cover plate 62firmly attached to rim 66 of port 60, the connection being sealed by Oring 68. Light bulb 70 is located in a receptacle 72 in cover plate 62to provide illumination for the interior of the furnace during loading,unloading and other periods when illumination is not provided bydischarge from the torch.

Also in the top of furnace shell 18 is arranged viewport 74 Whoseextending tube 76 is spanned by protective screen 78 and closed at itsouter end by window 80. The details of this construction are shown moreclearly in FIG. 3.

The hand-hole arrangement, by which articles inside the furnace may bemanipulated without affecting the furnace atmosphere seal, is shown indetail in FIG. 4 and FIG. 5. Handhole tube 82 is provided with gauntlet84, having a comparatively long sleeve 86 whose free end is fixedlyattached to the rim on tube 82 and in sealing engagement therewith.Gauntlet 84 may be fabricated of any suitable flexible and gasimpermeable material, such as nibber or plastic and its sleeve end maybe attached to the rim of tube 82 by a suitable adhesive and sealant.The end of tube 82 is closed and protected by cover 88, which ishingedly arranged to close the end of tube 82, as by articulated bolts90. Gauntlet 84 and its sleeve '86 are prevented from ballooning intothe interior of the furnace under the effect of vacuum therein byprovision of plate 92 which abuts against a ledge formed of spacedsegments 94 fixedly attached to the tube wall. When it is desired towork with gauntlet 84 inside the furnace, plate 92 is pivoted to aposition parallel with the axis of tube 82 so that it may he slippededgewise through the spaces between ledge segments 94 and placedtemporarily inside the furnace proper, manipulation being accomplishedthrough gauntlet 84. Plate 96 is normally maintained across tube 82 byaction of spring clips 98 to act as an additional radiation shield toprotect gauntlet 84 and sleeve '86 from heat in the furnace. Plate 96can be removed from its position as shown, conveniently when plate 92 isedgewise in the tube between ledge segments 94, and both plates may beplaced in the furnace temporarily and Without breaking the furnace sealby manipulation through gauntlet 84.

Mounted through the Wall of furnace shell 18 and sealed as it passestherethrough is a plasma jet torch, indioated generally at 100, whoseconstruction is shown in more detail in FIG. 6. Torch barrel 102slidably engages ball 104 being sealed as it passes therethrough byflexible bushing 106. Ball 104 is mounted in flexible cupped sealingring 108, which is in turn fixedly attached to mount tube assembly 110,as by screws 111, which act to tighten clamp ring 112. Torch 100 cantherefore be slidably moved up and down through seal bushing 106 and ina variety of pivotal movements through the action of ball 104 in itssealing ring 108. Material which is slightly flexible and which can becompressed to form a tight seal will be suitable for fabrication ofbushing 106 and ring 108. Preferably, these elements are fabricated froma plastic fluorinated hydrocarbon, such as manufactured and sold underthe trade name Teflon. Such material has self-lubricating properties, aswell as functioning adequately as a sealing material.

To the body 110 of plasma jet torch 100 is attached handle bar 113,attachment being accomplished by clamping ring 114. Handle bar 113 issuitably curved and bent so that it may be readily grasped by anoperator conveniently seated so that he can view the operation insidethe furnace through window 80 and also be in a position to perate thecontrol foot pedals mounted on horizontal bars 12.

One pole of a DC. electric power source is connected to the electrode ofplasma jet torch 100 (as will be seen more clearly in FIG. 6) by meansof power lead 116. Cooling water is supplied to the torch throughcooling pipe 118 connected to a suitable source of cooling fluid, suchas water, such fluid being withdrawn from torch 100 through drain line120 connected to a suitable sewer or sump, also not shown. Inert gasfitting 121 is connected to torch 100 to supply this gas for itsoperation.

The weight of plasma jet torch 100 is suitably counterbalanced so thatthe operator may readily actuate it up and down and rotatably to put theend of barrel 102 in proper position for melting metal in cavities 58 inhearth plate 52. Counter-balancing is arranged by provision of clamp122, which is fixedly attached to an upper part of plasma torch body110, conveniently supporting also Water pipes 118 and 120, as shown, theclamp being connected to cable 124, which passes over elevated pulleys126 and Whose free end is attached to counterweight 128.

Since the metal body being melted will, in most cases, also be connectedto the source of electric power, a suitable lead 130 is shown attachedto shell cover 18 and to plate 16 from which connection will be madethrough shaft 22 and to hearth 48 and its upper plate 52 and so to metalin a cavity 58. Various power levels are advantageous for best operationof the device, and these may be arranged by inclusion of suitable andconventional equipment in the power cabinet 45 and controlled throughlead 132 by pedal switch 134.

The furnace shell 18 will comprise a sealed, gas-tight shell in whichmetals may be melted without contact with atmospheric gases. Pipe 136fitted with valve 138 communicates with the furnace interior, as shown,and for initial evacuation may be connected to a suitable pump (notshown). After the furnace interior is evacuated, it may be back-filledwith inert gas through valved pipeline 139 to provide a protectiveatmosphere for melting. This atmosphere will in most cases be maintainedby additional inert gas introduced into the furnace shell through plasmajet torch 100. Excess gas may be vented from the furnace interiorthrough bleed pipe 140, whose outlet is submerged in oil bubbler 142,which prevents any back-flow or entry of outside atmosphere gases intothe furnace. Thus, after the furnace atmosphere is initiallyestablished, it is maintained automatically by the gas normallyintroduced by the plasma jet torch operation.

Plasma jet torch is of conventional design and is shown in FIG. 6 insuflicient detail for appreciation of its construction and operation inthe furnace of this invention. Cooling Water introduced in pipe 118circulates inside the body tube and also barrel 102 with baflie 144arranged spaced apart from inner wall 146 to provide a return forcooling water circulated back up to the top of the torch to bedischarged through pipe 120. The torch central electrode is formed witha tip 148 of heat an corrosion resistant metal such as tungsten, theelectrode proper being formed of an outer tube 150 and an inner tube152. Cooling water from pipe 118 is circulated down the electrodebetween tubes 150 and 152, being returned through inner tube 152 anddischarged through pipe 120. Inert gas is supplied to the torch throughfitting 121 from a convenient source (not shown) and provides a flow ofinert gas through the annular space 154 between the center electrode andthe inner barrel wall 146. When an arc is maintained between electrodetip 148 and metal in a mold cavity 58, the flow of inert gas isconverted to a very hot plasma eflluent directed out of the nozzle 156of torch 100 and which is caused to impinge on the metal to be melted.

In operation of the furnace of this invention, clamps 64 are firstloosened and then cover plate 62 is removed to open the furnace. Variouscharges of metals to be melted are then placed in cavities 58 in theupper plate 52 of hearth 48. The masses to be melted may be compacts ofpowdered or sponge metal, as, for example, titanium or zirconium spongeadmixed with various a1-= loying agents, or may be any other kind ofmetallic material which it is desired to melt into buttons or smallingots. Particular cavities 58a, 58b, or 580 will be selected, dependingon the desired shape of the metal button, pillow block, or bar.

The various charges being placed in the hearth cavities, cover plate 62is then replaced and securely sealed in position by retightening clamps64. Vacuum line 136 will have been previously connected to a suitablevacuum pump, which is now started, and valve 138 is opened so that theinterior of furnace shell 18 may be evacuated. Care will be taken thatthe valves in bleed line 139 and oil bubbler line -140 are closed andthat all other seals and fittings in and on the furnace are in propergas-tight engagement. Thegfurnace should be pumped down to a lowpressure indicating proper vacuum-tightness of the systemf-Withanlormal-sized button furnace of the type described, evacuation to 20microns pressure will indicate that the furnace is in proper conditionfor inert gas melting. After a vacuum-tightness check, a leak checkshould show no more than 5 microns per minute increase in pressure for a5-minute period; preferably the leak check should show no more than 2microns per minute for a 5-minute period.

After a satisfactory vacuum check, valve 138 is closed and a suitableinert gas, such as argon, is admitted into the furnace corivenientlythrough valve pipeline 139. With the furnace interior now back-filledwith argon, valve 139 is cloed and cooling water supply to plasma jettorch 100 is'turned on through inlet pipe 118, and argon gas supply isturned on through inlet pipe 121. Cooling water is" caused to becirculated through hearth 48 by inflow of water through pipe 28.Operation of pedal switch 134 now turns on power between the electrodein plasma, jet torchv 100 and the metal charge iii a cavity 58substantially jaligned underneath it. This produces a a plasma jetefiluent from the end of barrel 102, which may be accurately directedupon the metal charge by actuation of handle bar 113 to effect verticalor pivotal movement of toffch 100 as desired. After the charge in onecavity 58 is melted, the plasma jet torch power supply may be cut downmomentarily to a lower level. by operation of pedal switch 134, and thenhearth 48 is rotated by operation of pedal switch 42 so as to alignanother cavity 58 with its contained metal charge underneath the end ofplasnj'a torch barrel 102. With the new cavity in position, meltingpower is again applied to plasma jet torch 100 and efiiuent isaccurately directed to melt the new charge by pivoting or slidingmovement of plasma jet torch 100 bflactuation of handle bar 113. Whenthe second charge has been melted, the procedure may be repeated untilthe charges in the cavities 58 in hearth 48 have been initially melted.During melting, the argon atmosphere inside furnace shell 1'8 ismaintained by argon introduced into' plasma torch 94 as plasma-forminggas. Excess argon islvented from the furnace interior through oilbubbler 142.

After initial melting, and to insure homogenity in the buttons and smallingots, they are best remelted after turning over so that the bottomportions may be exposed to the torch melting heat. To accomplish this,power to plasma jet torch 100 is shut off and cover 88 of hand-hole tube82 is removed by loosening bolts 90, and the operator then inserts hishand in gauntlet 84 and first places plate 92 edgewise aril'tl thenplaces it and plate 96 temporarily inside the furnace shell 18. Thenwith his gauntlet hand he reaches inside the furnace shell 18 and turnsover the initially melted buttons or ingots. This turning is, as will beappreciated, accomplished Without admitting air into the interior of thefurnace.

After the buttons are turned, the operator, still with his hand workingin gauntlet 84, replaces plate 96 and 92. He then places the gauntlet 84and sleeve 86 in tube 82 extension and replaces cover 88 and tightensthis in place by means of bolts 90. Electric power is then appliedbetween the electrode in plasma jet torch 100 and one of the initiallymelted buttons or ingots in a cavity 58, and thismetal button or ingotis then remelted to form a final ingot. After remelting one such buttonor ingot, the hearth is rotated to align another underneath plasma jettorch 100, and the r'emelting operation is repeated as before. After.all the buttons and small ingots have been remelted (and if desiredremelting may be repeated several 6 times), plasma jet torch power isshut down by operation of pedal switch .134, and cover plate 62 isremoved after first loosening clamps 64. The finished buttons or ingotsmay then be removed 'from the furnace.

The meltingfurnace of this invention is convenient and efiicient inoperation. The plasma effluent from the torch is more stable than an arcdischarge heretofore employed for heating in such furnaces. Since theeffluent is directed from the end of the torch barrel as a stream ofplasma and gas, it can be pointed and accurately controlled toimpilfi'ge on the surface of metal in one of the hearth moldcavities.The means for directing this efiiuent, that is the slidable andpivotable mounting for the torch barrelfcnables the operator toaccurately control the torch effluent for best melting effect. Theplasma jet torch therefore provides a readily and accuratelycontrollable source; of intense heat for efficient melting of metalbodies successively in a plurality of mold cavities.

Another unique advantage of the furnace of this invention is theadditional feature of maintenance of an inert melting atmbsphere insidea sealed furnace shell using the inert gas supplied for operation of theplasma jet torch. I have found that a required amount of inert gas(according to norrnial operating procedures) for operation of the torchwill provide sufficient inert gas passing through the she'ffl', andvented without entry of atmospheric gases, to lfiiciently maintain adesirable inert gas melting atmosphere inside the furnace shell. Thisinsures that no deleterioufs oxidation or contamination by reaction withexternal atmospheric gases will occur. Thus, the inert gas supplied tothe torch serves a dual purposeit provides gas for" plasma generation,and also gas for maintenance of j a protective atmosphere inside thefurnace.

I claim:

1. A furnace for melting metals comprising:

(a) a sealed she'll;

(b) a rotatable: hearth in-said shell, said hearth having at least onemold cavity; v

(c) a plasma jet torch having a tip and a nozzle located within saidshell, and means to supply inert gas past said tip through said nozzleinto said shell;

(d) means adjustably mounting said torch for directing the efiluent fromsaid nozzle onto metal to be melted in a mold cavity in said hearth; and

(e) means for evacuating said shell.

2. A furnace for melting metals comprising:

(a) a sealed shell;

(-b) a rotatable hearth in said shell, said hearth having a plurality ofmold cavities;

(c) a plasma jet torch having a tip and a nozzle located within saidshell, and means to supply inert gas past said tip ;;.t hrough saidnozzle into said shell;

(d) means adjustably mounting said? torch for directing the effluentfrom said nozzle onto metal to be melted in a mold cavity in saidhearth; and

(e) means for evacuating said shell.

3. A furnace for melting metals comprising:

(a) asealed shell;

(b) a rotatable hearth in said shell, said hearth having a plurality ofmold cavities;

(c) a plasma jet torch having a tip and a nozzle located within saidshell, and means for supplying inert gas past said tip through saidnozzle into said shell;

(d) means slidably and pivotally mounting said torch in a wall of saidshell;

(e) means for rotating said hearth to position a selected one of saidplurality of mold cavities in position so that the efiiuent from saidnozzle is directed onto metal to be melted in said selected mold cavity;and

(f) means for evacuating said shell.

v 4. A furnace for melting metals comprising:

(a) a sealed shell;

(b) a rotatable, water-cooled horizontal hearth in said shell, saidhearth having a plurality of mold cavities;

(c) a plasma jet torch having a tip and a nozzle located within saidshell, and meansfor supplying inert gas past said tip through saidnozzle into said shell;

(d) means slidably and pivotably mounting said torch in a wall of saidshell;

(e) means for rotating said hearth to position a selected one of saidplurality of mold cavities in position so that the effluent from saidnozzle is directed onto metal to be melted in said selected mold cavity;and

('f) means for evacuating said shell.

5. A furnace as set forth in claim 4 including:

(f) a glass viewing window in said shell, and a metal screen interposedbetween the glass and the interior of said furnace shell.

References Cited 7 UNITED WILLIAM J. STEPHENSON, Primary Examiner. E.MAR, Assistant Examiner.

US. Cl. X.R.

